To be clear the Democratic Party is a private organization and they can do whatever they want. It’s completely within their rights to say “we need to stop Bernie Sanders” and put in action a plan to do just that.

Legally speaking, yes. Ethically, fuck no. If they admitted Bernie into the party so he could run in their primary, they created a huge chain of events that involved thousands of people and millions of dollars. It’s wrong for them to allow such a charade to go on if they have no intention of honoring their own voters.